Prior studies indicate that right brain function is altered in depressive disorders. Our long range goals have been to help clarify the nature of the right hemisphere abnormalities in different diagnostic subtypes of affective illness and to evaluate the effects of antidepressant medications on these abnormalities. We propose to extend and finish two ongoing studies that are using behavioral measures of perceptual asymmetry and event-related potential (ERP) measures of hemispheric asymmetry. Specifically, we aim: (1) to complete a comparison between unipolar patients vs. bipolar patients (including Bipolar I vs. Bipolar II) vs. normal controls in tandem with an atypical vs. typical distinction (see item 2); (2) to replicate in a new sample initial findings of differences in lateralized perceptual function between atypical depression (patients with features such as mood reactivity, hypersomnia or hyperphagia) and typical depression (patients without these features); (3) to test patients before and after treatment with antidepressants so as to determine whether alterations of lateral asymmetry on behavioral or ERP measures persist or disappear following treatment; (4) to evaluate whether pretreatment lateral asymmetries are predictive of therapeutic response to either a standard tricyclic antidepressant (TCA) or a monoamine oxidase inhibitor (MAOI); (5) to relate alterations of perceptual asymmetry in depressed patients to more direct ERP measures of hemispheric asymmetry; (6) to do a pilot study using measures of regional cerebral blood flow (rCBF). We propose to increase our sample sizes for Study 1 and Study 2 by testing an additional 30 unipolar depressed patients, 30 bipolar depressed patients (15 Bipolar I and 15 Bipolar II) and 30 normal controls. Study 1 uses a battery of dichotic listening and visual tachistoscopic measures of lateralized perceptual function. Study 2 simultaneously measures ERP measures of hemispheric asymmetry and behavioral measures of perceptual asymmetry in temporal and spatial discrimination tasks. A pilot study will measure the rCBF of 10 depressed patients and 20 normal controls in a resting condition and under activation conditions. Patients are tested after a minimum drug-free period of 10 days and are retested following 6 weeks of treatment with either a TCA, MAOI or placebo. The clinical significance of this research lies in the need for more precise diagnostic aids that facilitate the selection of antidepressants tailored to different types of depressive disorder.